The United States Thursday began a top-level diplomatic campaign on behalf of U.N. Secretary-General Kofi Annan's Cyprus settlement plan, which will be put before the voters of the Greek and Turkish Cypriot communities April 24.

The effort was led by President Bush, who telephoned Turkish Prime Minister Recep Tayyip Erdogan and his Greek counterpart Costas Caramanlis to urge them to use their influence with the Cypriot parties on behalf of the Annan plan, which a White House spokesman termed a "historic opportunity" to end the Cyprus dispute.

The U.N. secretary-general announced late Wednesday in Switzerland that the compromise plan will be put before the two Cypriot communities in referendums April 24 - this after Greek and Cypriot leaders ended six weeks of negotiations on the settlement package without a final agreement.

The United States has strongly backed Mr. Annan's effort to resolve the three-decade-long conflict, which among other things has been a major source of friction between U.S. NATO allies Greece and Turkey.

At a news briefing, State Department spokesman Adam Ereli said the outcome was necessarily a compromise in which no party got everything it sought. But he said the final text is fair and "meets the core interests" of both sides.

"This is a good settlement, and one that obviously involves compromise, but offers the best deal for both sides, and is something to be supported," he said. "It offers a solution to a problem that has bedeviled the international community for decades. We will be working with the secretary-general and others to encourage support for this plan. But obviously, it's up to the people of Cyprus."

Mr. Ereli said as they prepare to vote on the plan, Cypriots should know that the United States is committed to its full implementation, and is ready to make a "substantial contribution" at a preliminary donors conference being organized by the European Union to help meet the costs of the settlement.

If the plan is approved by the parties in the April 24 voting, a federation of two constituent Cypriot states would enter the European Union as a single entity on May 1. If the referendum fails, only the Greek Cypriot side would join the EU.

Cyprus has been divided since 1974, when Turkey occupied the northern third of the island in response to a coup in Nicosia aimed at union with Greece.

Under the Annan plan, a complex document of thousands of pages, territory under Turkish Cypriot control would be reduced from 37 percent to 29 percent of the island.

There would be provisions for the return or compensation of families forced from their homes in 1974, and the Turkish military presence would be phased down, eventually leading to the stationing of small Turkish and Greek garrisons on the island.

Both Greek and Turkish Cypriot leaders have expressed reservations about the plan, though objections appear to be stronger - and the fate of the referendum more doubtful - in the Greek Cypriot community.